Clever one this. The host bridge changes mode depending upon what version of the AGP standard the graphics card supports.

• AGP 2.0 and earlier
• AGP 3.5

  (13:57:27:davej@tetrachloride:davej)$ lspci -vvv -F kt400-agp3-pcixxx.txt 
  00:00.0 Host bridge: VIA Technologies, Inc. VT8377 [KT400 AGP] Host Bridge
  	Subsystem: Asustek Computer, Inc.: Unknown device 807f
  	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
  	Status: Cap+ 66Mhz+ UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ >SERR- <PERR-
  	Latency: 0
  	Region 0: Memory at f0000000 (32-bit, prefetchable)
  	Capabilities: [80] AGP version 3.5
  		Status: RQ=31 SBA+ 64bit- FW+ Rate=x1,x2
  		Command: RQ=31 SBA- AGP- 64bit- FW- Rate=<none>
  	Capabilities: [c0] Power Management version 2
  		Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
  		Status: D0 PME-Enable- DSel=0 DScale=0 PME-
  

• There's also another variant which seems a little closer to the KT266.

  00:00.0 Host bridge: VIA Technologies, Inc. VT8366/A/7 [Apollo KT266/A/333]
          Subsystem: VIA Technologies, Inc.: Unknown device 3189
          Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
          Status: Cap+ 66Mhz+ UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ >SERR- <PERR-
          Latency: 8
          Region 0: Memory at d0000000 (32-bit, prefetchable) [size=128M]
          Capabilities: [80] AGP version 3.5
                  Status: RQ=31 SBA+ 64bit- FW- Rate=x1,x2
                  Command: RQ=31 SBA- AGP- 64bit- FW- Rate=<none>
          Capabilities: [c0] Power Management version 2
                  Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                  Status: D0 PME-Enable- DSel=0 DScale=0 PME-
  

(17:55:42:root@mesh:agpgart)# ~/a.out 
version: 0.100
bridge id: 0x30991106
agp_mode: 0x1f000a0b
aper_base: 0xd0000000
aper_size: 128
pg_total: 23808
pg_system: 23808
pg_used: 0
entry.key : 0
entry.key : 1
Allocated 8 megs of GART memory
MemoryBenchmark: 976 mb/s
MemoryBenchmark: 1002 mb/s
MemoryBenchmark: 991 mb/s
Average speed: 989 mb/s
Testing data integrity (1st pass): passed on first pass.
Testing data integrity (2nd pass): passed on second pass.